This invention is generally directed to conductive carrier particles, and more specifically the present invention relates to processes for the preparation of conductive carrier particles, wherein the conductivity is, for example, from about 10.sup.-2 to about 10.sup.-10 (ohm-cm).sup.-1 by the dry coating of carrier cores with submicron conductive polymeric particles, comprised of a polymer or mixtures thereof and a conductive component, such as carbon black. Advantages associated with the present invention in embodiments include stable electrical characteristics, essentially the same carrier conductivity irrespective of the polymer coating weight, use of toxic solvents, and the recovery thereof can be eliminated, and the adverse effects of residual solvent on carrier conductivity is avoided, or minimized. In one embodiment, the process of the present invention comprises the preparation of conductive carrier particles by mixing submicron, less than 1 micron in average volume diameter for example, polymer particles containing carbon black, and applying by dry coating methods the resulting mixture to carrier cores of, for example, steel, iron, ferrites, and the like; and thereafter fusing by heating the polymer mixture to the carrier cores. The preparation of conductive polymeric particles with an average particle size diameter of from between about 0.05 micron to about 1 micron are illustrated in U.S. Pat. No. 5,236,629, the disclosure of which is totally incorporated herein by reference. The conductivity of the generated submicron polymeric composite particles can be modified by, for example, varying the weight percent of conductive filler component present in effective amounts of, for example, from between about 1 weight percent to about 50 weight percent, and also by varying the composition of the conductive filler component. Thus, conductive submicron polymeric composite particles with a conductivity of from between about 10.sup.-10 (ohm-cm).sup.-1 to about 10.sup.-4 (ohm-cm).sup.-1 can be prepared. In one process embodiment, the particles with average diameter of about 0.05 to about 1 micron of conductive composite particles are comprised of polymer and a conductive filler distributed evenly throughout the polymer matrix of the composite product, and which product can be obtained by a semisuspension polymerization method as illustrated in U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference. In the aforementioned semisuspension polymerization processes, a mixture of monomers or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component are bulk polymerized until partial polymerization is accomplished, for example. In one specific embodiment of the present invention, from about 10 to about 50 percent of monomers or comonomers are converted to polymer, thereafter the resulting partially polymerized monomers or comonomers are cooled to cease bulk polymerization and to the cooled mixture of polymerized monomers or comonomers is added carbon black, like REGAL 330.RTM., followed by mixing, using, for example, a high shear mixer until a homogeneous mixer, or organic phase is obtained. Subsequently, the resulting organic phase is dispersed in water containing a stabilizing component with, for example, a high shear mixer, then the resulting suspension is transferred to a reactor and completely polymerized; the contents of the polymerization reactor are then cooled, followed preferably be washing and drying the polymer product. The polymer product obtained can then be applied to a carrier core by the dry coating processes illustrated herein, reference U.S. Pat. Nos. 4,937,166 and 4,935,326, the disclosures of which are totally incorporated herein by reference.
Metals such as carrier cores are conductive or semiconductive materials, and the polymeric materials used to coat the surface of metals are usually insulating. Therefore, carrier particles coated completely with polymer or a mixture of polymers can lose their conductivity and become insulating. Although this is desired for some applications, for conductive magnetic brush systems (CMB) the carrier particles should be conductive. Since the carrier polymer coating can be utilized to control carrier tribo, a conductive carrier coating is needed to design carriers with the desired conductivity and triboelectrical properties. Conductive polymers are very costly, and are not considered suitable for preparing low cost, for example less than $5/pound, coating, thus a conductive polymer composite comprising a low cost polymer and a conductive filler, such as conductive carbon black, avoids these disadvantages.
A polymer composite coating of metal materials, such as carrier beads, can be obtained by two general approaches, solution and powder coating. Solution coating of carriers using a polymer composite solution comprised of a polymer, a conductive filler and a solvent can be utilized to prepare conductive carrier, however, trapping of solvent in the solution coating adversely interferes with the use of coated materials, for example the residual solvent trapped in the carrier coating reduces the carrier life, and the release of solvent in the developer housing can cause other problems related to harmful effects of absorbed solvent to various copying machine parts and toxicity of solvent. Moreover, the solvent recovery operation involved in the solution coating processes is costly. The powder coating of metal surfaces can eliminate the need for solvent, and therefore, many of the problems associated with solution coating; however, such coating requires polymer powder with a very small size, for example less than one micron. Although several polymer powders with desired particle size are available for carrier powder coating, submicron polymer composite particles containing conductive filler to prepare conductive coated carriers that maintain their triboelectrical characteristics for extended time periods exceeding, for example, 200,000 images are not known, or available. Therefore, there is a need for conductive submicron polymeric composite particles each containing a conductive filler distributed evenly throughout particles and processes for the preparation thereof.
The preparation of polymeric particles for powder coatings can be accomplished by three methods, namely grinding or attrition, precipitation and in situ particle polymerization. Grinding or attrition, especially fluid energy milling, of large polymeric particles or polymeric composite particles containing fillers to the size needed for powder coating, for example less than one micron, is often not desirable both from an economic and functional viewpoint. These materials are difficult to grind, and with present milling equipment is very costly due to very low processing yield, for example in the range of 5 to 10 weight percent. Precipitation process can also be used to prepare polymeric/polymeric composite particles. In one approach, the polymer solution is heated to above its melting temperature and then cooled to form particles. In another process, the polymer solution is precipitated using a nonsolvent, or the polymer solution is spray dried to obtain polymeric/polymeric composite particles. With all these precipitation processes, it has been difficult to achieve low cost and clean, that is for example with no or substantially no impurities such as solvents or precipitants in the resulting polymer, particles. It is also difficult to obtain particles with small particle size and narrow particle size distribution. Further, it can be difficult to control filler distribution throughout each particle's polymer matrix. In the in situ particle polymerization process, polymer particles are prepared by using suspension dispersion, emulsion and semisuspension polymerization. Suspension polymerization can be utilized to prepare polymer particles and polymeric composite particles containing, for example, a conductive filler. However, this process does not, for example, enable particles with a size less than five microns. Although emulsion and dispersion polymerization may be utilized to prepare polymeric particles of small size, for example less than one micron, processes wherein particle formation is achieved by nucleation and growth do not enable synthesis of particles containing fillers such as conductive fillers. Conductive fillers, such as carbon blacks, are free radical polymerization inhibitors terminating or at least reducing the rate of polymerization.
There is disclosed in U.S. Pat. No. 4,908,665 a developing roller or developer carrier comprised of a core shaft, a rubber layer and a resin coating layer on the surface of the rubber containing conductive fillers for a one component developer. It is indicated in the '665 patent that the conductive developing roller can eliminate variation of the image characteristic due to the absorption of moisture for one component development. This patent thus describes a developing roller for one component developer and does not disclose the preparation of conductive carrier beads for dry two component developer. U.S. Pat. No. 4,590,141 discloses carrier particles for two component developer coated with a layer of silicon polymer using fluidized bed solution coating. U.S. Pat. No. 4,562,136 discloses a two component dry type developer which comprises carrier particles coated with a silicon resin containing a monoazo metal complex. The two component carriers described in the above two patents are insulating, that is with a conductivity of less than 10.sup.-10 (ohm-cm).sup.-1 and are not believed to be conductive. There is disclosed in U.S. Pat. No. 4,912,005 a conductive carrier composition coated with a layer of resin containing a conductive particle by solution coating. Residual solvent trapped in the coated layer adversely effects the maintainability of carrier electrical properties of an extended time period.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles for fluidized bed powder coating are prepared by dispersing the polymer in a liquid which is heated to above the polymer melting point and stirred causing the polymer particles to form. The particles are then cooled below their melting point and recovered. However, this process does not, for example, effectively enable particles with a size of below 50 microns in diameter.
The suspension polymerization of monomer is known for the formation of polymer/polymeric composite particles generally in a size range of about 200 microns and higher. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets.
Further, disclosed in U.S. Pat. No. 4,835,084 is a method for preparing pigmented particles wherein a high concentration of silica powder is utilized in the aqueous phase to prevent coalescence of the particles. There is also disclosed in U.S. Pat. No. 4,833,060 a process for the preparation of pigmented particles by dissolving polymer in monomer and dispersing in an aqueous phase containing silica powder to prevent coalescence of the particles. However, the silica powder used in both U.S. Pat. Nos. '084 and '060 should be removed using KOH which is costly, and residual KOH and silica materials left on the surface adversely affect the charging properties of particles. Moreover, the above processes do not enable the preparation of submicron conductive particles. There is also disclosed in U.S. Pat. No. 3,954,898 a two step polymerization process for the preparation of a thermosetting finished powder. However, this process does not enable synthesis of particles with a size less than 100 microns. Moreover, this patent does not disclose the synthesis of submicron particles containing conductive fillers.
Disclosed in U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference, is a semisuspension polymerization process for the preparation of small polymeric particles which are comprised of a mixture of monomers or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component which are bulk polymerized until partial polymerization is accomplished. The resulting partially polymerized monomers or comonomers are dispersed in water containing a stabilizer component with, for example, a high shear mixer, then the resulting suspension polymerized, followed by washing and drying the submicron polymeric particles.
There remains a need for the preparation of carrier particles with submicron conductive polymeric particles, and more specifically conductive submicron polymeric particles containing conductive fillers distributed throughout each particle. Further, there is a need for a dry coating process to obtain carrier particles with conductive submicron polymer particles, each containing conductive fillers evenly distributed in the polymer, and more specifically, there is a need for conductive carrier particles that contain a polymer and carbon black prepared by semisuspension polymerization processes and wherein there is obtained low cost, clean and dry small, for example from between about 0.05 to about 1 micron in average diameter as determined by a scanning electron microscope, polymeric particles containing from about 1 to about 50 weight percent of a conductive filler, such as carbon black, which is evenly distributed throughout the polymer matrix.